Ultra-violet radiation-induced modifications to undoped metal-organic chemical vapor-deposited nitrides on sapphire were observed at temperatures of between 9 and 160K. The photoluminescence intensities of the bound excitons (3.476, 3.482eV), the yellow band (2.2eV) and the blue band (2.9eV) changed with time when a fresh sample was irradiated with 325nm He-Cd laser light. The free exciton peak at 3.488eV was unchanged by laser irradiation. The blue and donor-bound exciton emission initially deteriorated rapidly and the yellow luminescence increased; both at the same rate. The yellow luminescence deteriorated and the donor-bound exciton emission later increased very slowly; at the same rate. A model for the short-time behavior was expected to involve the initial presence of carriers of one type (such as electrons) on so-called blue metastable centers which offered a barrier to the capture of further electrons. It was tentatively assumed that blue-band emission was outside of the dislocations that were associated with the yellow band in the bulk region. At longer times, it was suggested that the space-charge regions surrounding dislocations repelled further carriers, thus leading to the decrease in the yellow band. A lack of total recovery of emission at room temperature, particularly after many cycles, suggested that photochemical or recombination-enhanced defect reactions occurred.

Reversible Ultraviolet-Induced Photoluminescence Degradation and Enhancement in GaN Films. B.Kim, I.Kuskovsky, I.P.Herman, D.Li, G.F.Neumark: Journal of Applied Physics, 1999, 86[4], 2034-7